Help me pleaseee geometry help

If cos(2x) = tan^2(y), show that cos(2y) = tan^2(x).

The difference of three and the reciprocal of a number is equal to the quotient of 20 and the number. find the number.

Which of the following is the major negative aspect of crossover designs for research studies? a. prohibitive cost b. residual effects c-subject drepout d. incomplete randomization e. large sample size required